1. Technical Field
This invention relates to a method and system for diagnosing a secondary air supply used in an internal combustion engine and more particularly to non-invasive methods and systems adapted to provide an indication of whether threshold emission levels are being exceeded.
2. Background
As is known in the art, an internal combustion engine emits exhaust gas consisting of products from the combustion of the air/fuel mixture added to the engine. Fuel is a mixture of chemical compounds, termed xe2x80x9chydrocarbonsxe2x80x9d (HC). The various fuel compounds are a combination of hydrogen and carbon. Under perfect combustion conditions, the hydrocarbons would combine in a thermal reaction with the oxygen in the air to form carbon dioxide (CO2) and water (H2O). Unfortunately, perfect combustion does not occur and in addition to CO2 and H2O, carbon monoxide (CO), oxides of nitrogen (NOx), and hydrocarbons (HC) occur in the exhaust gas as a result of the combustion reaction. Additives and impurities in the fuel also contribute minute quantities of compounds such as lead oxides, lead halogenides, and sulfur oxides. Therefore, federal statutes have been enacted to regulate the allowable amount of HC, NOx, and CO emitted from a vehicle""s engine.
The greatest effects on the combustion process, and therefore on the exhaust emissions, is the mass ratio of air to fuel. The air/fuel ratio must lie within a certain range for optimal ignition and combustion. For an internal combustion engine, the mass ratio for complete fuel combustion is approximately 14.7:1; i.e., 14.7 kilograms of air to 1 kilogram of fuel. This ratio is known as the stoichiometric ratio. In terms of volume, approximately 10,000 liters of air is required for 1 liter of fuel.
When the fuel mixture contains excessive fuel, or is running rich, CO emissions increase almost linearly with the increasing amount of fuel. However, when the fuel mixture contains excessive oxygen, or is running lean, CO emissions are at their lowest.
As with CO emissions, HC emissions increase with an increasing amount of fuel. At very lean air/fuel ratios, the HC emissions increase again due to less than optimal combustion conditions resulting in unburned fuel.
The effect of the air/fuel ratio on NOx emissions is the opposite of HC and CO on the rich side of stoichiometry. As the air content increases, the oxygen content increases and the result is more NOx. However, on the very side of stoichiometry, NO x emissions decrease with increasing air because the decreasing density lowers the combustion chamber temperature.
To reduce the exhaust gas emission concentration, a catalytic converter is typically installed in the exhaust system of an internal combustion engine. Chemical reactions occur in the converter that transform the exhaust emissions to less harmful chemical compounds. The most commonly used converter for an internal combustion engine is the three-way converter (TWC). As the name implies, it simultaneously reduces the concentration of all three regulated exhaust gases: HC, CO, and NOx. The catalyst promotes reactions that oxidize HC and CO, converting them into CO2 and H2O, while reducing NOx emissions into N2. In order for the catalytic converter to operate at the highest efficiency for conversion for all three gases, the average air/fuel ratio must be maintained within less than 1% of stoichiometry.
Typically, automobile manufacturers utilize an exhaust gas oxygen sensor in the electronic engine control system to maintain stoichiometric air/fuel ratio. This sensor is installed in the exhaust system upstream of the catalytic converter and responds to the oxygen content in the exhaust gas. The oxygen content is a measure of the excess air (or a deficiency of air) in the exhaust gas. The output of the sensor is a measure of the air/fuel ratio of the exhaust gas. Automobile manufacturers also utilize a secondary air pump to reduce the emission of CO and HC. The air pump is controlled by the electronic engine controller (EEC).
Currently, the air pump turns on during engine idle specifically for testing the functional operation of the secondary air system. More particularly, during idle, fuel into the engine is controlled and the air pump is turned on. An exhaust gas oxygen sensor disposed upstream of the catalyst is used to detect whether the secondary air system is operating properly. This invasive method requires special fueling conditions that may increase engine emissions.
In an effort to improve engine emission performance, new legal requirements require that the secondary air system performance be monitored while the pump is active at startup (i.e., a non-invasive system) and to notify the driver when secondary air system performance degrades to the point where vehicle emissions exceed a predetermined threshold.
A method is provided for diagnosing operational performance of a secondary air system used in an internal combustion engine. The engine produces exhaust gases. The exhaust gases pass through a catalytic converter. The engine includes an exhaust gas oxygen sensor. The secondary air system introduces air into the exhaust gas after engine start-up. The oxygen sensor produces a feedback control signal during a feedback mode for adjusting engine operating air-fuel ratio, such feedback mode being initiated at a first time. The method determines from the signal produced by the exhaust gas oxygen sensor a second time relative to the first time when the oxygen sensor signal indicates that a predetermined exhaust gas air fuel ratio is produced. The operational performance of the secondary air system is diagnosed as a function of the determined second time.
With such method a non-invasive secondary air supply diagnostic method is provided. Further, the method is able to notify the driver when system performance degrades to the point where vehicle emissions exceed a predetermined threshold.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.